The delay of the button appearance is 3.9 seconds on the right, and 4.0 seconds on the left. The difference is 0.1 seconds, but in fact the difference between them is about 1 second.

[Forge] v3.3.2 MC 1.20.1

The calculation should be correct. This is probably just some combination of background shenanigans delaying the loading and the opacity being too low to see the element at the beginning.

If it was just background shenanigans, it would occur randomly, right? But no. I've tried this a few times. Whenever I set the value to 4.0, the delay is much higher than when I set 3.9. If you try 3.8, 3.7, 3.6 and so on, then the delay is correct, and there is no such big difference.

The single-player button has a delay of 3.9 seconds, and all other buttons have a delay of 4.0 seconds.
https://github.com/user-attachments/assets/b22619fb-b2a1-46e8-8843-9a5bbc418f56

I also tested it without all the mods and it still works. Only Konkrete (1.8.0), Melody (1.0.2) and Fancy Menu (3.3.2) are enabled

The delay is incorrectly calculated only on versions 3.3.0 and higher. On version 3.2.3, everything is calculated correctly

You were right, it was caused by a calculation error.

It also seemed to me that the fade speed began to be calculated the other way around compared to previous versions.